community: add issue/PR templates, code of conduct, update contributing guide

.github/pull_request_template.md

@@ -0,0 +1,15 @@
+## What does this PR do?
+
+<!-- Brief description of the change -->
+
+## Related issue
+
+<!-- Link to issue, e.g. Fixes #123 -->
+
+## Checklist
+
+- [ ] `cargo fmt` passes
+- [ ] `cargo clippy` passes with no warnings
+- [ ] `cargo test --lib --bin nora` passes
+- [ ] New functionality includes tests
+- [ ] CHANGELOG.md updated (if user-facing change)